1. Field of the Invention
This invention relates to apparatus for feeding stock material intermittently to a press and more particularly to press feed apparatus that includes a pair of dual driven feed rolls mounted for movement of one feed roll toward and away from the other feed roll and operable to intermittently feed stock material to a press.
2. Description of the Prior Art
In automatic punch press operations, and particularly presses that operate at high speed, a strip of stock material is fed from a coil to the dies of the press for punching, stamping, cutting or the like. A preselected length of the stock material is fed to the press after each press operation. The stock material is fed from the coil in timed relation with the press operation so that before the dies contact the stock material, the stock material is moved into a final position by the die pilots as the feed rolls are released from engagement with the stock material. The stock material is stationarily positioned between the dies during the press operation. After the press operation, the feed rolls are returned to a position engaging the stock material. The driven feed roll is then actuated to rotate through preselected angle of rotation to advance another length of the stock material to the press. The feeding of the stock material to the press must be coordinated with each press operation so that prior to each operation, a new segment of stock material is in position relative to the dies for the press operation.
U.S. Pat. Nos. 4,133,216 and 4,138,913 are examples of punch press feeding apparatus in which a driven feed roll is intermittently rotated through a 36O.degree. rotational cycle. During the rotational cycle, the driven feed roll and a idler feed roll advance a preselected length of the stock material to the press. During a dwell period of the rotational cycle, the driven feed roll is not rotated and the press operation is performed. After the press operation is completed, another preselected length of stock material is advanced to the press by the intermittent rotation of the driven feed roll.
With the above described prior art arrangement, the length of stock material incrementally advanced to the press is determined by the angle of rotation of the driven feed roll. By increasing and decreasing the angle of rotation of the driven feed roll, the feed length of the stock material to the press is adjusted. When the press operation is performed, the driven feed roll and the idler feed roll are removed from driving engagement with the stock material and the stock material is engaged by a clamp. During this interval, the driven feed roll is released from engagement with the stock material and rotated in the opposite direction of the direction of feed to return the driven feed roll to the initial feed position. Thereafter, the driven feed roll and the idler feed roll are returned to engagement with the stock material and the clamp is released.
The driven feed roll is incrementally rotated and a preselected length of the stock material is fed to the press. Movement of the stock material is transmitted to the idler roll, resulting in rotation of the idler roll. A problem frequently encountered with this arrangement is slippage of the stock material between the driven feed roll and the idler roll. One proposed solution to preventing slippage of the stock material between the rolls is increasing the clamping engagement of the feed roll and the idler roll with the stock material when the feed and idler rolls are returned to feeding engagement with the stock material. However, an additional problem is encountered when the clamping force is increased, particularly with relatively thin stock material or easily deformable stock material. Stock material of this type is susceptible to deformation and misregister of a stamping or cutting pattern. If the clamping force for relatively thin stock material is excessive, the stock material becomes elongated and the feed length for each stamping operation is not maintained uniformly. Therefore, there is need for press feeding apparatus that incrementally feeds the workpiece to the press by oscillatory movement of a pair of driven feed rolls where each roll is driven at the same degree of rotation and the rolls are movable into and out of feeding engagement with the stock material.